This map, created for The Lowdown by Bay Area web developer Victor Powell, shows fluctuations since 2010 in California’s 30-largest reservoirs. As of July 30, many are at strikingly low levels — even in light of typically bone dry summer conditions — leaving water agencies throughout the state scrambling for options, and beginning to impose water-use restrictions and penalties.

On the map, the grey border marks each reservoir’s maximum capacity, while the blue shows fluctuating storage levels over time. All reservoirs are scaled relative to maximum storage capacity by area (with the largest — Shasta Lake — at 30 pixels). Click on each reservoir to see the change in capacity over the designated time period. Changing percentages can be viewed by hovering over the reservoir pop-out to the left of the graph. Use the slider at top to select specific times. The source code and data set is available here.

California is peppered with a vast collection of dams and reservoirs, providing water — and in some cases power — to cities and farms throughout the state.

Most of the state’s reservoirs typically fill up during the wet winter months and get slowly depleted over the course of the summer and fall (although reservoirs used mainly for agriculture and power generation follow scattered release schedules).

But these aren’t typical times for California: following several below-normal years of snowfall and rain, 2013 clocked in as the state’s driest year on record. And the winter months of 2014 provided little relief, leaving the state bracing for drought conditions into the foreseeable future.